1. Field of Invention
The present invention relates to, in general, a fuel pump and, in particular, one for a direct-injection system.
2. Description of Related Art
A direct-injection system comprises a plurality of injectors, a common rail that feeds the fuel under pressure to the injectors, a high-pressure fuel pump that feeds the fuel to the common rail through a high-pressure feeding conduit and is provided with a flow-rate-adjustment device, and a control unit that pilots the flow-rate-adjustment device for keeping the fuel pressure inside the common rail equal to a desired-value generally time-course variable as a function of the operating conditions of the engine.
The high-pressure fuel pump described in Patent Application EP2236809A1 comprises a pumping chamber in which a piston slides with alternating motion, a suction channel regulated by a suction valve for feeding the low-pressure fuel inside the pumping chamber, and a delivery conduit regulated by a delivery valve for feeding the high-pressure fluid outside the pumping chamber and toward the common rail through the feeding conduit.
The suction valve is normally controlled under pressure and, in the absence of external actions, closed when the fuel pressure inside the pumping chamber is higher than that in the suction channel and open when the fuel pressure inside the pumping chamber is lower than that inside the suction channel. The flow-rate-adjustment device is mechanically coupled to the suction valve to keep, when necessary, the suction valve open during pumping of the piston and, thereby, allow the fuel flow to exit from the pumping chamber through the suction channel. In particular, the flow-rate-adjustment device comprises a control rod that is coupled to the suction valve and movable between a “passive” position, in which it allows the suction valve to close, and an “active” position, in which it does not allow the suction valve to close. The flow-rate-adjustment device comprises further an electromagnetic actuator that is coupled to the control rod for moving the control rod between the “active” and “passive” positions. The electromagnetic actuator comprises a spring that keeps the control rod in the “active” position and an electromagnet that is adapted to move the control rod to the “passive” position by magnetically attracting a ferromagnetic anchor integral with the control rod against a fixed magnetic armature.
It has been noted that, in use, the high-pressure fuel pump described in Patent Application EP2236809A1 produces a noise similar to a ticking that can be clearly perceived when the engine is at low revolution speeds (i.e., overall noise generated by the engine is poor). The noise generated by the high-pressure fuel pump can be clearly perceived also because, since the high-pressure fuel pump must take the motion from the driving shaft, it is directly mounted onto the engine head a motor head of which transmits and spreads the vibration generated by the high-pressure fuel pump.
The noise produced by the high-pressure fuel pump in use is essentially due to the cyclical impacts of the movable equipment of the flow-rate-adjustment device (i.e., control rod and the anchor) against the suction valve and magnetic armature of the electromagnet. To reduce such noise, it has been proposed to act via software on the intensity and waveform of the piloting current of the electromagnet to minimize the kinetic energy of the movable equipment upon the impact against the suction valve and magnetic armature. It has been experimentally noted that, by acting via software on the piloting current of the electromagnet, it is possible to considerably reduce the kinetic energy of the movable equipment upon the impact against the magnetic armature. Conversely, it has been experimentally noted that, by acting via software on the piloting current of the electromagnet, it is much more complex and expensive to considerably reduce the kinetic energy of the movable equipment upon the impact against the suction valve.
To considerably reduce the kinetic energy of the movable equipment upon the impact, the control system must energize the electromagnet with a piloting current that is as close as possible to the “limit” piloting current (which imparts the “minimum” kinetic energy to the movable equipment upon the impact). But, above all, the control system must energize the electromagnet with a piloting current that never drops below the “limit” piloting current, or the actuation is lost (i.e., movable equipment never reaches the desired position due to insufficient kinetic energy). The value of the “limit” piloting current is highly variable according to the case because of the construction leakages and drifts due to time and temperature. In the case of impact against the magnetic armature, the control system is facilitated since the reaching of the “limit” position (i.e., performance of the actuation) may be verified by observing the fuel pressure inside the common rail (when the control rod impacts against the magnetic armature, the suction valve closes and, thus, the high-pressure fuel pump starts pumping fuel under pressure, which increases the fuel pressure inside the common rail). Therefore, the control system can progressively decrease the piloting current until the reaching of the “limit” position (i.e., performance of the actuation) disappears. And, at this point, it can slightly increase the piloting current for carrying out the actuation with the “minimum” kinetic energy upon the impact. On the other hand, in the case of impact against the suction valve, there is no way to check the reaching of the limit position (i.e., performance of the actuation), and, thus, the control system must completely act in open ring, being definitely ineffective in limiting the kinetic impact energy and, therefore, noise.
Thus, there is a need in the related art for a fuel pump for a direct-injection system. More specifically, there is a need in the related art for such a fuel pump that is free from the above-described drawbacks and simple and inexpensive to make.